Metal Alkyls in Initiations of Cationic Polymerizations
Initiations of polymerizations of vinyl and other monomers by metal alkyls generally take place by anionic mechanisms. This is discussed further in this chapter. There are, however, reports in the literature [41] of cationic polymerizations that are initiated by some metal alkyls. These are polymerizations of monomers like vinyl ethers, o- and p-methoxystyrene [42], and isobutylene [43, 44] that are initiated by compounds like dialkyl aluminum chloride.
One explanation is as follows [45]. These metals are strong electron acceptors. Their valence shells and their unfilled orbitals can accommodate electrons from donor molecules. As a result, the organometallic compounds behave like Lewis acids. This was observed in a polymerization of isobutyl vinyl ether [46]. While triethylaluminium is not by itself an initiator for the polymerizations, the reactions will take place in the presence of typical electron acceptors [45]:
The catalyst components must be combined in the presence of the monomer. This is due [45] to high instability of the carbon cation complexes [Al(C2H5); CI] ̄ [CH2OCH2] *. It is interesting that the polymerizations by C2HAICI2 H2O also show rate decreases as the reaction progresses [47]. When, however, additional water is added, rapid polymerizations start again [47].
It is possible to modify the catalytic activity of metal alkyls by controlled additions of modifiers. These are water, alcohol [45], oxygen [48], carbon dioxide [49], aldehydes [50], organic peroxides [45], and metal oxides, like V2Os, NiO, and HgO [45]. The exact action of these modifiers is not clear and it is not certain if they should be considered to be co-initiators or initiators. Their addition, however, can affect catalytic activity, yield, stereospecificity, and molecular weight of the products. Oxygen, for instance, can act as a modifier for the Grignard reagent, which by itself does not initiate cationic polymerizations of vinyl ethers. Yet, introduction of oxygen to a vinyl ether-Grignard reagent system will initiate the polymerization and will yield high molecular weight products [50]. It was suggested [45] that oxygen may cause transformation of the alkyl magnesium groups into N alkoxy magnesium groups. This results in greater concentrations of magnesium dihalides that can induce the polymerizations.
Substances that generate cations can vary widely. They can be molecules that dissociate into ions or react with other compounds like solvent or monomer to form cations. Iodine is an example of such a substance. In a system of n-butyl vinyl ether-iodine-diethyl ether, the iodine apparently first forms an inactive л-complex with the solvent. It subsequently dissociates and rearranges into an isomeric active ion [12]:
Another group of compounds that were recently reported as capable of initiating cationic polymerizations are metallocene/borate complexes. Such material can, for instance, be generated from zirconocene dimethyl compounds (Cp2ZrMe2) and anilinium borate. Thus, [HNMe,Ph] + [B(C6H5)4] will polymerize amine-functionalized a-olefins [51] as well as isobutylene homo and copolymerizations [52]. Also, when compounds, like Cp*MMe3 (M = Ti, Zr, and Hf; where Cp* = ns-pentamethylcyclopentadienyl) are reacted with B(C6F5)3, a methyl cation is abstracted and a complex forms, as shown below:
Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, N-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical n'-fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53]:
